Project background

Blumeria graminis f. sp. hordeii (BGH) is a powdery mildew fungus which infects barley which can result in reduced crop yields of up to 40% in temperate regions. The lifestyle of the fungus is extremely unusual in that it is an obligate biotroph, capable of only growing on living plant cells, and shows a high degree of host-specificity. The project aimed to gain a better understanding of factors influencing infection and the unusual lifestyle of the fungus. The project was a collaboration between Imperial College London (Prof. Pietro Spanu), INRA (Marc-Henri Lebrun, Joelle Amselem), The John Innes Centre (James Brown), The University of Exeter (Prof. Nick Talbot) and the University of Oxford (Prof. Sarah Gurr).

Powdery mildew fungus penetrating through the wall of a barley epidermal cell

The BDSG was heavily involved in the full lifetime of the project from the initial grant-writing through to submission of the final results to the public databases. The project funded a post-doctoral fellow with the BDSG for three years (Dr. Tim Burgis), in addition to a BDSG half-post (Dr. James Abbott) for the same period. The BDSG carried out the genome assemblies using novel hybrid methodologies to integrate conventional Sanger-based sequence with high-throughput sequence data, along with integration of cDNA, EST and proteomic resources. The genome annotion was manually curated through a community-based process, drawing heavily on the knowledge of powder mildew experts from around the world. The group of scientists brought together through these annotation efforts continue to meet regularly and collaborate to this day. The culmination of the project has resulted in the inclusion of the genome assembly in the Ensembl Genomes database.

Outcomes

Isolated haustorium: the fungal feeding structure is surrounded by a plant membrane. The haustorium is also the main site of secretion of fungal effectors that control host immunity.

The genome sequence revealed a number of surprises about the fungus. It was widely anticipated that the lifestylye of BGH, tightly integrated with it's host, would result in a reduced genome size, since certain resource could be obtained from the host rather than needing to produce them itself. In contrast, the genome turned out to be around three time larger than anticipated, with the main size increase due to the presence of a large number of novel repetitive DNA sequences. Analysis of the gene content of the genome indicated that some of the pathways which normally help protect against such repetitive DNA sequences occurring are missing in BGH, resulting in a high number of gene duplications. A downstream study of the genome sequence of effector sequences, which are thought to be involved in host-immunity and disease resistance characterised a superfamily consisting of 491 genes, making up 7% of the total gene complement of the fungus.